Proboscis adaptations for flower feeding

Butterflies use a proboscis, a microfluidic probe engineered by natural selection, to feed on nutritive fluids. The structural configuration of proboscises relates to feeding habits; however, the adaptations that enable proboscis entry into narrow floral corollas lack experimental evidence.
Here, we investigated proboscis adaptations that enable entry into corollas using funnel-shaped glass capillary tubes and performed feeding trials with six butterfly species of different feeding habits. Proboscises were either guided (natural treatment) or forced (forced treatment) into the capillary tubes that were filled with a 20% sucrose solution. The treatments were video recorded to determine the depth the proboscises reached into the tube and how long they remained there. The results were interpreted in terms of proboscis morphology, friction forces, and the material properties of the cuticle.
In the natural treatment, butterflies classified as flower visitors were more efficient at feeding from the tubes, reaching an average 1.83x deeper into the tubes than the other species and never getting their proboscises stuck. The non-flower visiting species, in contrast, had their proboscises remain in the tube 17x longer than the flower-visiting species, with 90% of them getting their proboscises at least partially stuck. The butterfly species with generalist feeding habits fed more efficiently than the non-flower visitors, but less than the flower visitors. A similar pattern was observed in the forced treatment.
Flower-visiting butterflies had smoother and more tapered proboscises, lower friction forces, and a semicircular cross-section that would reduce bendability and was augmented by a more sclerotized cuticle. Proboscises of flower-visiting butterflies, therefore, have a suite of adaptations that operate synergistically to optimize their feeding habits.